mirror of
https://github.com/dolthub/dolt.git
synced 2026-04-23 21:59:01 -05:00
Dhruv Sringari
92 lines
2.6 KiB
Go
92 lines
2.6 KiB
Go
// Copyright 2022 Dolthub, Inc.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package nbs
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import (
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"encoding/binary"
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"errors"
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"io"
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)
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var (
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ErrNotEnoughBytes = errors.New("reader did not return enough bytes")
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)
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func NewIndexTransformer(src io.Reader, chunkCount int) io.Reader {
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tuplesSize := chunkCount * prefixTupleSize
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lengthsSize := chunkCount * lengthSize
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suffixesSize := chunkCount * addrSuffixSize
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tupleReader := io.LimitReader(src, int64(tuplesSize))
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lengthsReader := io.LimitReader(src, int64(lengthsSize))
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suffixesReader := io.LimitReader(src, int64(suffixesSize))
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return io.MultiReader(
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tupleReader,
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NewOffsetsReader(lengthsReader),
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suffixesReader,
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)
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}
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// OffsetsReader transforms a byte stream of table file lengths
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// into a byte stream of table file offsets
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type OffsetsReader struct {
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lengthsReader io.Reader
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offset uint64
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}
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func NewOffsetsReader(lengthsReader io.Reader) *OffsetsReader {
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return &OffsetsReader{
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lengthsReader: lengthsReader,
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}
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}
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func (tra *OffsetsReader) Read(p []byte) (n int, err error) {
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// Read as many lengths, as offsets we can fit into p. Which is half.
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// Below assumes that lengthSize * 2 = offsetSize
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// Strategy is to first read lengths into the second half of p
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// Then, while iterating the lengths, compute the current offset,
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// and write it to the beginning of p.
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// Align p
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rem := len(p) % offsetSize
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p = p[:len(p)-rem]
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// Read lengths into second half of p
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secondHalf := p[len(p)/2:]
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n, err = tra.lengthsReader.Read(secondHalf)
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if err != nil {
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return 0, err
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}
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if n%lengthSize != 0 {
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return 0, ErrNotEnoughBytes
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}
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// Iterate lengths in second half of p while writing offsets starting from the beginning.
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// On the last iteration, we overwrite the last length with the final offset.
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for l, r := 0, 0; r < n; l, r = l+offsetSize, r+lengthSize {
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lengthBytes := secondHalf[r : r+lengthSize]
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length := binary.BigEndian.Uint32(lengthBytes)
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tra.offset += uint64(length)
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offsetBytes := p[l : l+offsetSize]
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binary.BigEndian.PutUint64(offsetBytes, tra.offset)
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}
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return n * 2, nil
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}
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